The aim of this study was to investigate the percentage of regulatory T cells and the correlation with clinical activity in Behçet's disease. Forty Behçet's disease (BD) patients, 18 males and 22 females, were included in the study. The patients were diagnosed according to the published BD criteria. Twenty age- and sex-matched volunteer healthy donors were also included. At the time of sampling, clinical activity was assessed for activity signs and symptoms according to the BD Current Activity Form. We considered active those patients with a clinical activity index equal to and/or above 2. Eleven patients had active disease and the remaining were considered as clinically inactive. We have studied the percentages of CD4+CD25+FOXP3+, CD4+CD25+ and CD4+FOXP3+regulatory T cells by flow cytometry and investigated the correlation with disease activity. Percentage of CD4+CD25+FOXP3+Treg in active patients was lower than clinically inactive patients and healthy controls. Percentage of CD4+CD25+Treg was not different between active and inactive patients and healthy controls. Percentage of CD4+FOXP3+Treg was lower than healthy controls in clinically active patients but was not different for inactive group and healthy controls. Patients were active when CD4+CD25+FOXP3+Treg was ≤1.19 %, CD4+CD25+Treg was ≤2.68 %, and CD4+FOXP3+Treg was ≤2.60. CD4+CD25+FOXP3+Treg and CD4+FOXP3+Treg were found negatively correlated with disease activity. Peripheral blood regulatory T cells are decreased in clinically active Behçet's disease patients. The advances in our understanding of the interactions between distinct subsets of Treg and clinical activity might help in modulating BD treatment.